1. Field of the Invention
This invention relates to an image forming apparatus such as a printer, a copier or a facsimile apparatus utilizing the electrophotographic method or the electrostatic recording method.
2. Related Background Art
Many of image forming apparatus utilizing the electrophotographic method adopt a contact type charging method and a contact type transferring method which are low in the production of harmful ozone, and above all, in the transferring portions thereof, the mainstream is a roller transferring method excellent in the convey ability of a transferring material such as paper.
Now, the roller transferring method is a method of urging a transferring roller having an elastic rubber roller layer against an electrophotographic photosensitive body (hereinafter referred to as the photosensitive drum) which is an image bearing body to thereby form a transfer nip, and transferring a toner image on the photosensitive drum to the transferring material by the action of a transfer bias applied to the transferring roller while conveying the transferring material by the transfer nip. As the transferring roller, use is generally made of an elastic sponge roller of hardness 20 to 40 degrees (Asker-C) comprising a mandrel of SUS, Fe or the like and an electrically semiconductive sponge elastic layer formed thereon and having its resistance adjusted to 1xc3x97106 to 1xc3x971010xcexa9 by a carbon or ion electrically conductive filler or the like.
However, a transferring roller having a diameter about half that of the photosensitive drum has heretofore been used. Accordingly, even in a transferring roller in which the ratio of the transferring roller diameter/the photosensitive drum diameter (hereinafter referred to as the diameter ratio) is small, for example, the diameter of the transferring roller is 14 mm relative to the diameter of 30 mm of the photosensitive drum, and the diameter ratio is 14/30xe2x89xa10.47, and in many apparatuses, the diameter ratio is 0.5 or greater. This is because in the manufacture of the transferring roller, electrically semiconductive foamed rubber is polished after formed around a mandrel, whereby the diameter thereof is adjusted, and the reason for polishing is that the accuracy of xc2x10.1 mm or less is required because the transferring roller affects the conveyability of the transferring material.
In the method of adjusting the diameter of the transferring roller by polishing, there has been the problem that a grinder traverses in the axial direction of the roller and therefore, if the rigidity of the mandrel is low, the mandrel is flexed and a diameter difference occurs between the end portion and center of the roller and thus, the direction of conveyance differs between the center and end portion and the transferring material becomes wrinkled.
Also, to increase the restraining force at the nip portion of the transferring roller in order to eliminate the blur of an image, Asker-C hardness of 40 degrees or less is required, and for this reason, a method of foaming rubber and thickening the elastic rubber roller layer has been adopted.
However, when transfer is effected in a state in which the diameter ratio is great, there has been the problem that when the transferring material comes into the transfer nip and when it is discharged therefrom, the scattering of the image occurs under the influence of the electric field of the transferring roller and the gradation of the halftone is aggravated or the line image becomes blurred.
It is an object of the present invention to provide an image forming apparatus in which a toner image is little scattered.
It is another object of the present invention to provide an image forming apparatus which can use a transferring roller of a small diameter.
It is still another object of the present invention to provide an image forming apparatus comprising:
a roller-shaped image bearing member bearing a toner image thereon; and
a transferring roller pressure-contacted with the image bearing member for transferring the toner image on the image bearing member to a transferring material, a radius of the transferring roller being 12 mm or less;
wherein
r/R less than 0.4,
where r is the radius of the transferring roller, and R is the radius of the image bearing member.
Further objects of the present invention will become apparent from the following description.